With the progressive development in the field of application of powder coating materials in recent years, it has been expected eagerly to realize improvements in the visual quality of the coatings obtained from such powder coating materials. Under these circumstances, there have been proposed several powder coating materials in which an aesthetic appearance is provided by a surface irregularity in the resulting coating film.
For example, Japanese Patent Publication No. 7686/1973 discloses a powder coating material in which a powder of a resin which is solid at room temperature and has an average particle size of not higher than 200 -mesh and a melt viscosity of, at the highest, 100 poise at 100.degree.-250.degree. C., on the one hand, and a powder of a resin which is solid at room temperature and has an average particle size in the range from 60-mesh to 200-mesh and a melt viscosity of, at the highest, 100 poise at 100.degree.-250.degree. C., on the other hand, are mixed together. The coating layer formed from this powder coating material exhibits a poor aesthetic appearance of an orange peel-like pattern due to its shallow profile of surface irregularity.
In the Japanese Patent Application Kokai No. 46739/1975, a powder coating material is disclosed which is prepared by kneading a mixture composed of a powder of a thermosetting resin which is solid at room temperature and a powder of a thermoplastic resin based on a cellulose ester at such a temperature that the thermosetting resin will melt but the cellulose ester resin will not, cooling the kneaded mass and then finely pulverizing it. However, this powder coating material suffers from an insufficient distribution of the cellulose ester resin over the resulting mass of the coating material since the two powder products of the thermosetting resin and the thermoplastic cellulose ester resin are kneaded at the same temperature, so the coating layer obtained therefrom will not only reveal an unevenly distributed crepe-like pattern but also give rise to a phenomenon of such an excessive shrinkage that the substrate becomes laid bare at the "valley" portions of the wrinkling of the coating, resulting in an inferior corrosion resistance.
In the TECHNICAL DATA SHEET of the company TROY CHEMICAL, it is described that TROYKYD PTA (trademark) has been brought into market as additives for powder coating materials which provide an aesthetic appearance to the resulting coatings. However, the actual compositions of these additives are not given and, in addition, a sufficient distribution of such an additive will not be attained when added to a powder coating material due to the difficulty of attaining a uniform mixing of the two pulverous products with each other, so that the coating layer obtained therefrom may suffer from a disadvantageous phenomenon in which the substrate becomes exposed at the "valley" sites of the wrinkling coating, resulting in that not only a beautiful uniform pattern of the coating cannot be obtained but also a rusting of the substrate may occur when a ferrous substrate is used.
Furthermore, there is disclosed, in Japanese Patent Application Kokai No. 258270/1985, a powder coating material prepared by kneading a powder mixture comprising a thermosetting resin which is solid at room temperature and a thermoplastic resin based on a cellulose ether, at such a temperature that the thermosetting resin melts but the thermoplastic cellulose ether resin does not, cooling the resulting kneaded mass and then finely pulverizing it. However, this powder coating material suffers from an insufficient distribution of the cellulose ether resin over the resulting mass of the coating material, since the two powder products of the thermosetting resin and the thermoplastic cellulose ether resin are kneaded at the same temperature, so that the coating layer obtained therefrom will not only reveal an unevenly distributed wrinkle pattern but also gives rise to a phenomenon in which the substrate becomes laid bare at the "valley" sites of the wrinkling of the coating, resulting in an inferior corrosion resistance.